mem: Make the coherent crossbar account for timing snoops

This patch introduces the concept of a snoop latency. Given the
requirement to snoop and forward packets in zero time (due to the
coherency mechanism), the latency is accounted for later.

On a snoop, we establish the latency, and later add it to the header
delay of the packet. To allow multiple caches to contribute to the
snoop latency, we use a separate variable in the packet, and then take
the maximum before adding it to the header delay.

1 files changed, 16 insertions, 0 deletions

diff --git a/src/mem/coherent_xbar.cc b/src/mem/coherent_xbar.cc
index 52f69e0c9..32ca6d02d 100644
--- a/src/mem/coherent_xbar.cc
+++ b/src/mem/coherent_xbar.cc
@@ -187,6 +187,8 @@ CoherentXBar::recvTimingReq(PacketPtr pkt, PortID slave_port_id)
     Tick packetFinishTime = clockEdge(Cycles(1)) + pkt->payloadDelay;
 
     if (!system->bypassCaches()) {
+        assert(pkt->snoopDelay == 0);
+
         // the packet is a memory-mapped request and should be
         // broadcasted to our snoopers but the source
         if (snoopFilter) {
@@ -204,6 +206,10 @@ CoherentXBar::recvTimingReq(PacketPtr pkt, PortID slave_port_id)
         } else {
             forwardTiming(pkt, slave_port_id);
         }
+
+        // add the snoop delay to our header delay, and then reset it
+        pkt->headerDelay += pkt->snoopDelay;
+        pkt->snoopDelay = 0;
     }
 
     // forwardTiming snooped into peer caches of the sender, and if
@@ -377,9 +383,15 @@ CoherentXBar::recvTimingSnoopReq(PacketPtr pkt, PortID master_port_id)
     // we should only see express snoops from caches
     assert(pkt->isExpressSnoop());
 
+    // set the packet header and payload delay, for now use forward latency
+    // @todo Assess the choice of latency further
+    calcPacketTiming(pkt, forwardLatency * clockPeriod());
+
     // remeber if the packet is inhibited so we can see if it changes
     const bool is_inhibited = pkt->memInhibitAsserted();
 
+    assert(pkt->snoopDelay == 0);
+
     if (snoopFilter) {
         // let the Snoop Filter work its magic and guide probing
         auto sf_res = snoopFilter->lookupSnoop(pkt);
@@ -398,6 +410,10 @@ CoherentXBar::recvTimingSnoopReq(PacketPtr pkt, PortID master_port_id)
         forwardTiming(pkt, InvalidPortID);
     }
 
+    // add the snoop delay to our header delay, and then reset it
+    pkt->headerDelay += pkt->snoopDelay;
+    pkt->snoopDelay = 0;
+
     // if we can expect a response, remember how to route it
     if (!is_inhibited && pkt->memInhibitAsserted()) {
         assert(routeTo.find(pkt->req) == routeTo.end());